diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index abab88a9c6c3..82f6fae4a034 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1512,6 +1512,10 @@ ASTDeclReader::VisitRedeclarable(Redeclarable<T> *D) {
 template<typename T>
 void ASTDeclReader::mergeRedeclarable(Redeclarable<T> *D, 
                                       RedeclarableResult &Redecl) {
+  // If modules are not available, there is no reason to perform this merge.
+  if (!Reader.getContext().getLangOptions().Modules)
+    return;
+  
   if (FindExistingResult ExistingRes = findExisting(static_cast<T*>(D))) {
     if (T *Existing = ExistingRes) {
       T *ExistingCanon = Existing->getCanonicalDecl();
diff --git a/clang/test/Modules/redecl-merge.m b/clang/test/Modules/redecl-merge.m
index 4ea982b50fee..ab264d1c5b0d 100644
--- a/clang/test/Modules/redecl-merge.m
+++ b/clang/test/Modules/redecl-merge.m
@@ -1,6 +1,6 @@
 // RUN: rm -rf %t
-// RUN: %clang_cc1 -fmodule-cache-path %t -I %S/Inputs %s -verify
-// RUN: %clang_cc1 -x objective-c++ -fmodule-cache-path %t -I %S/Inputs %s -verify
+// RUN: %clang_cc1 -fmodules -fmodule-cache-path %t -I %S/Inputs %s -verify
+// RUN: %clang_cc1 -x objective-c++ -fmodules -fmodule-cache-path %t -I %S/Inputs %s -verify
 @class C2;
 @class C3;
 @class C3;